1. Technical Field of the Invention
The present invention relates to a novel topically applicable cosmetic/dermatological compositions for the photoprotection of human skin and/or hair. The subject compositions prevent and/or to combat aging of the skin, especially due to ultraviolet A radiation, and protect the skin and/or the hair against such radiation and against free radicals. The compositions of the invention are formulated, for example, as smooth white creams or as gels which may be topically applied to the human face, body and/or legs and to the hands and scalp.
2. Description of the Prior Art
It is known to this art that, over time, various signs which are very characteristic of aging appear on the skin, reflected in particular in a modification of the cutaneous structure and functions.
The principal clinical signs of cutaneous aging are, notably, the appearance of deep wrinkles and fine lines, which increase with age. In particular, disruption of the "grain" of the skin is observed, namely, its microrelief is less uniform and is anisotropic in nature.
Moreover, the skin complexion is generally modified; it appears paler and yellower, which appears to be due essentially to disruption of the microcirculation (less haemoglobin in the dermal papillae). Furthermore, many colored and/or darker blemishes appear on the skin surface, and more especially on the hands, imparting heterogeneity to the skin. In general, these blemishes are due to considerable production of melanin in the skin epidermis and/or dermis. In certain instances, these blemishes may become cancerous. Thus, it is increasingly sought to reduce these blemishes, or even to eliminate same. Too, diffuse irritations, and sometimes telangiectasia, may occur on certain areas of the skin.
Another clinical indication of aging is the dry and rough appearance of the skin, which is essentially due to a more considerable descuamation; by scattering light rays, these squama also contribute to the somewhat grey appearance of the complexion.
Finally, a loss of firmness and of tonicity of the skin is observed which, as in the case of wrinkles and fine lines, is at least partly explained by a dermal and epidermal atrophy and flattening out of the dermoepidermal formation; the skin is thinner and more flaccid, and the thickness of the epidermis decreases.
It is thus observed that the clinical signs of cutaneous aging result essentially from a dysfunction of the principal biological mechanisms taking place in the skin.
Such aging can be physiological in nature but also photoinduced, namely, due to repeated exposure of the skin to sunlight, and especially to ultraviolet A irradiation. The action of this light on the constituents of the skin and on the sebum secreted by the skin results, in particular, in the formation of oxygenated free radicals. These free radicals inflict considerable damage, especially in cell membranes (permeability of the membranes), cell nuclei (mutation by action on RNA or DNA) and tissues (necrosis and degeneration); it is thus necessary to protect the skin against these free radicals.
Hence, serious need continues to exist in this art for compositions capable of preventing and/or combating the onset of aging and the signs of aging, such as wrinkles and fine lines, of preventing and/or combating skin pigmentation blemishes, whatever their origin, and of protecting the skin, especially by suppression of the formation of oxygenated free radicals.
One of the known means for effectively combating premature aging of the skin is to topically apply thereto a UVA screening agent which absorbs at wavelengths between 320 and 400 nm. This decreases the excess photoinduced free radicals. However, such photoprotection is less than complete with the majority of compositions containing UVA screening agents. Thus, upon repeated exposures, the residual amount of free radicals, persisting despite the protection by the UVA screening agent, can induce, over the long term, photoactinic aging phenomena.
It has been proposed to increase the amounts of UVA screening agents, but it is not advisable to apply excessively large levels of screening agents in cosmetic products for daily care. Indeed, with the majority of screening agents, a maximum protection factor is often attained which is very difficult to improve by increasing the level of screening agents. In addition, it is necessary to take account of the fact that daily care products are more frequently used than other seasonal products containing screening agents, such as sunscreen products for example, which can result in cosmetic discomfort and problems of toxicity by the topical route.
In order to more effectively combat photoactinic aging, it is therefore advantageous to employ means other than the use of large amounts of UVA screening agent. In particular, it is possible to combine, with UVA screening agents, molecules capable of blocking the chain reactions of free radicals before the final stages of degradation of the biological constituents of the skin. These compounds are antioxidizing agents and/or anti-free radicals.
It too is known that photoinduced free radicals arise principally from molecular oxygen. Given that it is common in the body and that it is readily able to accept electrons, the free radicals and the activated oxygen species which derive therefrom are the most numerous participants in radical reactions. The following are representative:
Singlet oxygen: non-radical, high oxidizing, very toxic and very rare because it has a very short lifetime. It is the product of the excitation of molecular oxygen by light photons.
The superoxide radical anion: it is the product of the addition of an electron to molecular oxygen. It can initiate the production of very reactive free radicals, the hydroxyl radicals.
Hydrogen peroxide: non-radical but which can initiate the production of hydroxyl radicals.
The hydroxyl radical: it is highly oxidizing, and therefore very reactive, and the most toxic in respect of human cells.
Exemplary thereof are lipoperoxide radicals, which are species derived from the oxidation of membrane lipids.
Extracellular iron is also representative, which, by reacting with hydrogen peroxide and the superoxide radical anion which have accumulated outside the cell, will promote the production of the hydroxyl radical.